High voltage processes have been widely used in power management integrated circuits (PMIC's) and switch mode power supplies (SMPS's). For example, an SMPS has a high-voltage start-up circuit that operates with a high input voltage, such as a voltage in a range of about 40 V to about 600 V. In particular, recently, green power is more desired in various types of applications. Green power requires a higher conversion efficiency and a lower standby power consumption. To satisfy such requirements, a high-voltage junction-gate field-effect transistor (HV JFET) is usually used in the high-voltage start-up section of a circuit, such as a switch mode power IC.
An HV JFET usually includes a deep N-type well (deep NWell, an N-type well that has a relatively large depth as compared to a conventional JFET) or a high-voltage N-type well (HV NWell, an N-type well that has a relatively low doping level as compared to a conventional JFET) to serve as a channel between a source and a drain. However, since the doping level in an HV NWell is relatively low, variations during a process for making the HV JFET using an HV NWell can significantly affect a pinch-off voltage of the HV JFET. For example, the pinch-off voltage of the HV JFET may shift by about 25% due to variations in a well implantation step or a dopant drive-in step.